Ah, a specific variant of the Continuum Fallacy, applied to probability. Yep.
I'd still be somewhat surprised if it didn't have its own name yet. But if it doesn't, I suppose we can create a good neologism. What name should it have as a particular variant? (The way argumentum ad Hitlerum or argumentum ad cellarium are argumentum ad hominem variants.) Does anything snappy spring to mind?
What's wrong with "fallacy of gray"?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.